Silicon nitride (SiN) films are widely used in semiconductor devices and ultra-large-scale integrated circuits. For example, SiN films have been widely used in semiconductor devices as a diffusion barrier for dopants, as an etch-stop film during etching of fine features, as a final passivation film for encapsulation of fabricated devices, among many other uses.
SiN films can be deposited at low pressure or at atmospheric pressure using a variety of processing systems and process gases. These processing systems can perform, for example, thermal chemical vapor deposition (TCVD), plasma-enhanced chemical vapor deposition (PECVD), or remote-PECVD, where in remote-PECVD the substrate to be processed is not placed in direct contact with the plasma but is placed down-stream of the plasma discharge, among others. Device quality SiN films have been deposited, for example, by PECVD using silane (SiH4) and ammonia (NH3) or nitrogen (N2) or thermal CVD using dichlorosilane (SiH2Cl2) and NH3.
Deposited SiN films are often under stress. The stress can be either compressive or tensile, and can vary depending on the deposition process, gas mixture, deposition rate, substrate temperature, hydrogen content of the SiN film, ion bombardment or other process parameters. Tensile stress greater than about 1 GPa has been observed for SiN films. In PECVD deposition, ion bombardment of the SiN film can be used to densify films and induce more compressive stress. High tensile stress of a SiN passivation film can result in high stress between the SiN passivation film and the underlying substrate. In one example, capping a negative metal oxide semiconductor (NMOS) device containing a gate stack with a high tensile stress SiN film has been shown to induce tensile channel strain in the NMOS structure, thereby increasing electron mobility and the speed of the device. It has been observed that the amount of bonded hydrogen in SiN films decreases during electron bombardment and/or annealing in proportion with tensile stress increase.